Hitherto, a variety of hydrogen occluding alloys have been proposed, and a hydrogen occluding alloy which is disclosed on page 369 of the abstract of "The 35th Battery Symposium of Japan" held in November, 1994, in Nagoya-shi, has particularly attracted attention to battery electrodes.
The hydrogen occluding alloy is a Ni-based alloy having a reduced composition comprising, by wt % (hereinafter "%" indicates "wt %"),
33.2% of rare earth elements essentially consisting of La and/or Ce, PA1 9.8% of Co, 1.9% of Al, PA1 5.2% of Mn, and PA1 the balance being Ni and unavoidable impurities; and having a single phase CaCu.sub.5 -type crystal structure. PA1 32 to 38% of rare earth elements, PA1 0.5 to 3.5% of Al, PA1 0.5 to 10% of Mn, PA1 0.005 to 0.5% of hydrogen, and PA1 the balance being Ni and unavoidable impurities; PA1 wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure in a ratio of 0.5 to 20% by area. The aforementioned rare earth elements preferably comprise La and/or Ce, optionally together with other rare earth elements including Pr and Nd. This alloy may optionally further contain Co in an amount in the range of from 0.1 to 17 wt %. PA1 32 to 38% of rare earth elements, PA1 0.1 to 17% of Co, PA1 0.5 to 3.5% of Al, PA1 0.5 to 10% of Mn, PA1 0.005 to 0.5% of hydrogen, and PA1 the balance being Ni and unavoidable impurities; PA1 wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure in a ratio of 0.5 to 20% by area. The aforementioned rare earth elements preferably comprise La and/or Ce, optionally together with other rare earth elements including Pr and Nd. PA1 32 to 35% of rare earth elements, PA1 0.5 to 3.5% of Al, PA1 0.5 to 10% of Mn, PA1 0.005 to 0.2% of hydrogen, and PA1 the balance being Ni and unavoidable impurities; PA1 wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure in a ratio of 0.5 to 10% by area. The aforementioned rare earth elements preferably comprise La and/or Ce, optionally together with other rare earth elements including Pr and Nd. PA1 32 to 35% of rare earth elements, PA1 4 to 17% of Co, PA1 0.5 to 3.5% of Al, 0.5 to 10% of Mn, PA1 0.005 to 0.2% of hydrogen, and PA1 the balance being Ni and unavoidable impurities; PA1 wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure in a ratio of 0.5 to 10% by area. The aforementioned rare earth elements preferably comprise La and/or Ce, optionally together with other rare earth elements including Pr and Nd. PA1 melting raw materials in weight percentages substantially corresponding to the weight percentages in the alloys of the present invention as discussed above, thereby forming an alloy; PA1 casting the molten metal, thereby forming an ingot; PA1 (optionally) temper-annealing the alloy by heating the alloy to a temperature of from about 850 to 1050.degree. C. (referred to herein as a "temper-annealing" step); PA1 then subjecting the alloy to a hydrogen atmosphere of a pressure in the range of from 1 to 2 atms (atmospheres), preferably 1 to 1.2 atms, and a temperature in the range of from about 0 to 100.degree. C. (referred to herein as the "holding step"); PA1 then heating the alloy to a temperature in the range of from 600 to 950.degree. C., preferably 700 to 900.degree. C., (referred to herein as the "heating step"); PA1 and then cooling the alloy (referred to herein as the "cooling step"), thereby providing an alloy having a novel microstructure in which fine rare earth element hydride is dispersively distributed in a CaCu.sub.5 -type crystal matrix, with rare earth element at the Ca sites, and e.g., Ni, Co, Al, and Mn at the Cu sites.
Hydrogen occluding alloy is typically made by preparing a molten alloy having a given composition and casting it into an ingot. When putting it to practical use as a battery electrode, for example, the ingot is subjected to temper annealing in a vacuum or nonoxidizing inert gas atmosphere at a given temperature between 900 and 1,050.degree. C. for a given time period, if necessary, and the as-cast or temper-annealed ingot is mechanically pulverized to a predetermined particle size or pulverized by a hydrogenation process under a pressurized hydrogen atmosphere which includes hydrogen absorption at a given hot temperature between 10 and 200.degree. C. and hydrogen desorption by vacuum evacuation.
In addition, when the hydrogen occluding alloy is applied to, for example, a battery electrode, the battery can serve a practical use after an initial activation treatment in a pressurized hydrogen atmosphere for a given time period until the electrode including the hydrogen occluding alloy has a sufficient discharge capacity at an initial stage of use.